-Lactam antibiotics represent the largest and clinically most used family of antibacterial agents. The existence of -lactamases, enzymes capable of destroying -lactams, in Gram-negative bacteria is a serious clinical problem. A subset of these enzymes, carbapenemases, hydrolyzes carbapenem antibiotics avidly. Carbapenems are drugs of last resort in treatment of life-threatening infections. Enzymes with carbapenemase activity have been identified in all four molecular classes of -lactamases (classes A, B, C and D). Class D carbapenemases are widely distributed in Acinetobacter and Enterobacteriaceae isolates that are resistant to other available classes of antibiotics, and these carbapenemase-producing bacteria are often multi-drug or even pan-drug resistant. Infections caused by such pathogens are difficult or sometimes impossible to treat. In this grant proposal we will perform detailed microbiological, kinetics and structural characterization of the major class D carbapenemases of A. baumannii. These enzymes are widely distributed in clinical A. baumannii isolated and have elevated this pathogen to the category of most deadly and difficult to treat, with mortality rates often exceeding 30-50%. We will elucidate the mechanism of carbapenemase activity of A. baumannii CHDLs and evaluate the evolutionary potential of these class D enzymes to further enhance their catalytic efficiency against carbapenems, rendering this important class of antibiotics obsolete for treatment of infections caused by this bacterial pathogen. Finally, we will design and test inhibitors of class D carbapenemases that would restore clinical utility of carbapenem antibiotic for combating deadly A. baumannii infections.